Many women struggling to have a baby turn to in-vitro fertilization to improve their chances, and then face further uncertainty and anxiety when confronted with the decision of whether to use frozen or fresh embryos. A new study by a researcher at Yale School of Public Health and co-authors in China finds an insignificant difference in live birth rates between the two methods.

In this study, published in the New England Journal of Medicine, Heping Zhang of the Yale School of Public Health (YSPH) collaborated with scientists from throughout China and monitored 2,157 women who were undergoing their first in-vitro fertilization cycle and were randomly assigned either fresh or frozen embryos.

Researchers found that women using frozen embryos had a live birth rate of 48.7%, versus a live birth rate of 50.2% for women in the fresh-embryo group. There were also no significant differences between the groups in rates of implantation, clinical pregnancy, overall pregnancy loss, and ongoing pregnancy.

The study, however, did find that frozen-embryo transfers resulted in a significantly lower risk of ovarian hyperstimulation syndrome (OHSS) than women using fresh-embryo transfers. While most cases of the syndrome are minor, severe cases can lead to illness and even death. Women using frozen embryos were found to have a 0.6% chance of developing OHSS, versus a 2% chance for women receiving fresh embryos.

“This is an emerging issue of immediate and important concern for couples who are seeking in-vitro fertilization treatment,” said Zhang, the Susan Dwight Bliss Professor of Biostatistics at YSPH.

Reports by the Society for Assisted Reproductive Technology have noted that more fertility centers choose to defer embryo transfer by freezing embryos, Zhang said. To the researchers’ knowledge, this is the first time that a large clinical trial was conducted to evaluate this change of practice.

Zhang said that the findings will immediately impact patients and clinicians in their evaluation and consideration with regard to the risk and benefit of different embryo transfer strategies.

“This study also exemplifies the success of a multidisciplinary and international collaboration in addressing emerging and clinically important issues that patients and clinicians face as the technologies quickly evolve,” he said.

The study was funded by the National Key Research and Development Program of China and the National Natural Science Foundation of China.